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Current Experiments Archive

E08-025: Measurement of the Deeply Virtual Compton Scattering off the Neutron

This experiment is part of a program of measurements to obtain tomographic (3-D) images of the quarks inside the proton and neutron. In the experiment, electrons from the CEBAF accelerator will be directed into liquid hydrogen and deuterium targets in Hall A. The electrons will interact with the protons and neutrons inside the target nuclei. The scattered electrons will then be detected by one of the two High Resolution Spectrometers.

When the electrons transfer a sufficiently large amount of their momentum to a proton or neutron, the “Deeply Virtual Compton Scattering (DVCS)” reactions, eγ → epγ and en → eng, measure the distribution of quarks inside protons or neutrons as functions of their momenta along one axis and their spatial distribution in the plane perpendicular to this axis.

The DVCS reaction can occur through two processes, named “Bethe-Heitler” and “Compton” after the original authors. In both, a photon (γ) is emitted from either the electron or the proton. Each process features a distinct angular pattern of the emitted photons. The emitted photons (which carry off roughly half the energy of the incident electron) will be detected in a custom array of lead-fluoride (208 PbF2) crystals.

The specific goal of these experiments is to measure Deeply Virtual Compton Scattering at three incident electron energies (corresponding to three, four and five passes of the electrons through the CEBAF accelerator). The relative importance of the “Bethe-Heitler” and “Compton” amplitudes changes with the incident energy. In this way, the intensities of the interference and the pure Compton process can be measured (the Bethe-Heitler contribution can be calculated).